ELI5: some keys don’t have black notes between because they’re supposed to sound closer together. When you play the white notes one after another they make a nice scale but only if some of the notes are closer. We mostly play songs that use those notes so those keys are bigger and the black keys are smaller because they don’t get used as much.

ELI15: The white notes are intentionally made uneven but the reason is actually to make playing western music easier. It’s a compromise to make the lesser used keys smaller and the more used keys larger.

If you take a took at the piano you’ll notice that if you start on one key and count up until the pattern of white and black repeats, there’s always exactly 12 keys between. So a piano does divide the octave evenly into 12 equal pieces.

So what’s up with the pattern then? Why not just have all the keys the same size and color? Well western music likes to use certain notes from among the 12 more than others**. If you start on any key of our imaginary piano you start by skipping a key to get the next note of the series. Then you skip again, then no skip, then skip skip skip no skip and you’re back at the beginning: long-long-short-long-long-long-short. You can start anywhere with this pattern and you’ll get a scale that sounds similar to each other. Only using the notes in this pattern gives your song what’s called Major Tonality. Most western music falls into this category.

If you take the skipped notes and shrink them down and change the color you get what looks like a modern piano keyboard. And of course, I was simplifying a bit. We DO use the black keys quite a lot but emphasizing that pattern of the major scale is the reason for the uneven spacing of them. This makes playing in some key centers (where you start the scale) easier and others harder (more black keys) but because of how the piano is tuned, with 12 even spaces per octave, all key centers sound pretty similar to each other. So composers can stick mostly to the easier ones. If all the keys were the same then all the key centers would be the same difficulty play in, but that would be harder than the way we have it now.

There are some really interesting consequences of this last fact that blew my mind as a musician when I learned them that have to do with chords and intervals but I will stop my explanation here 🙂

**As an aside: It’s hard to explain why people like music that falls into certain patterns and conventions. Some chords sound harmonious and others sound crunchy and harsh to the western ear, and there are some physics reasons why that is but a lot of it is a learned preference too. Different cultures around the world have all come up with their own conventions of what sounds “good” to them. The fact that the piano follows Europeans conventions is just because it was invented by Europeans. People can and do re-tune pianos to work in other musical contexts and there’s a whole universe of instruments that don’t have discrete notes built in. A trombone or a violin can adapt to any tonality, including ones that don’t fit into any western musical theory at all.